In electrophotographic image formation, an electrostatic latent image that corresponds to the image is formed on a photoreceptor surface. An electric charging process, in which the photoreceptor surface is uniformly charged, is needed prior to the formation of the electrostatic latent image. The photoreceptor can be charged by one of two schemes: the non-contact charge scheme and the contact charge scheme.
The non-contact charge scheme involves the use of a so-called “corotron charger”, a so-called “scorotron charger” or the like. These chargers induce corona discharge, which in turn supplies electric charge to the photoreceptor through the air. Since the charger does not contact the photoreceptor in the non-contact charge scheme, the photoreceptor is less likely to be contaminated or wear out, which are advantages of the scheme. On the other hand, the scheme has a problem that the corona discharge entails ozone and other byproducts.
A recent trend which has emerged due to consideration of the environment is contact chargers which do not involve any corona discharge. Some of them employ a rubber member to contact the photoreceptor. The member is shaped like a roller, and voltage is applied to it. The roller that includes the rubber member is generally called a charging roller.
Document 1 discloses an arrangement in which a ten-point height of irregularities Rz of a surface of a charging roller is set to 10 μm or less. This arrangement is intended to stably remove deposits from the surface thereof for a long term and to sufficiently suppress the occurrence of poor charging or uneven charging. Document 1 also discloses an arrangement in which a lubricant such as a zinc stearate is applied to the photoreceptor, in the image forming apparatus using this charging roller. The lubricant is applied to improve performance of cleaning a residual toner on the photoreceptor after image-transfer and to prevent wear-out of the photoreceptor.
Documents 2 to 9 disclose a technology of subjecting a rubber member of a charging roller to surface processing. The technology disclosed in these documents modifies (hardens) a surface of the rubber member made of an epichlorohydrin rubber base material by treating the surface with an isocyanate compound. This modification makes it possible to prevent, for example, an ionic conductive agent from seeping from the surface, without an additional layer formation around the rubber member.
Document 1: Japanese Unexamined Patent Publication No. 189509/2005 (Tokukai 2005-189509) (published on Jul. 14, 2005)
Document 2: Japanese Unexamined Patent Publication No. 281830/1993 (Tokukaihei 5-281830) (published on Oct. 29, 1993)
Document 3: Japanese Unexamined Patent Publication No. 346051/2000 (Tokukai 2000-346051) (published on Dec. 12, 2000)
Document 4: Japanese Unexamined Patent Publication No. 348443/2001 (Tokukai 2001-348443) (published on Dec. 18, 2001)
Document 5: Japanese Unexamined Patent Publication No. 40760/2002 (Tokukai 2002-40760) (published on Feb. 6, 2002)
Document 6: Japanese Unexamined Patent Publication No. 82514/2002 (Tokukai 2002-82514) (published Mar. 22, 2002)
Document 7: Japanese Unexamined Patent Publication No. 191960/2004 (Tokukai 2004-191960) (published on Jul. 8, 2004)
Document 8: Japanese Unexamined Patent Publication No. 191961/2004 (Tokukai 2004-191961) (published on Jul. 8, 2004)
Document 9: Japanese Unexamined Patent Publication No. 53544/2006 (Tokukai 2006-53544) (published on Feb. 23, 2006)